diff --git a/lib/mongoid_spacial/spacial.rb b/lib/mongoid_spacial/spacial.rb
index 397fadd..5aef7bb 100644
--- a/lib/mongoid_spacial/spacial.rb
+++ b/lib/mongoid_spacial/spacial.rb
@@ -12,6 +12,7 @@ module Spacial
       :m  => EARTH_RADIUS_KM*1000,
       :mi => EARTH_RADIUS_KM*0.621371192, # taken directly from mongodb
       :ft => EARTH_RADIUS_KM*5280*0.621371192,
+      :sm => EARTH_RADIUS_KM*0.53995680345572, # sea mile
     }
 
     RAD_PER_DEG = Math::PI/180